1. Field of the Invention
The present invention is directed to a device and method for the continuous high speed and high current electric resistance heating and annealing of wires, and more particularly, to a constant current surgical needle annealing device and method.
2. Discussion of the Prior Art
Surgical needles are formed from wires cut to provide needle blanks. After processing one end of the needle blank to a desired needle shape, excess portion of the needle blank is cut. Prior to cutting the excess needle blank, the portion to be cut is annealed or heated. This softens the portion to be cut to facilitate cutting and further processing, such as forming a channel or drilling a hole in the soft portion for suture insertion.
Annealing may be performed using flames, conventional ovens, induction heating or resistance heating. U.S. Pat. No. 4,295,033 (Lindgren) discloses an annealing oven. Conventional annealers suffer from a number of disadvantages, such as limited accuracy, consistency and speed. Some flame annealers are limited to rack or manual operations. Open flames, including a pilot light, increase the danger of accidents, oxidize the needle, and deposit carbon and soot on the needle. Further, heat sinks may be required to confine the anneal zone. Heat transfer from flame annealing is inconsistent due to variations in flame temperature, gas pressure, and flame application time or traverse rate. Moreover, there is no feedback or indication of proper or improper annealing. In addition, the speed of annealing is low, thus reducing the overall needle manufacturing output.
Various types of flame annealers have been used to anneal a specific portion of a wire or needle blanks. Flame annealers may be used in a continuous needle forming process where needle blanks are mounted on a carrier strip 15, as shown in FIG. 1. Alternatively, flame annealers are used with a batch needle forming process. In batch needle forming processes, needles are formed in batches. A batch of needle blanks are manually arranged on a fixture or rack for processing. The fixture may be an integral part of the annealing mechanism. Alternatively, the fixture may have its own free standing mechanism which is attached to the annealing mechanism.
A motor may control exact positioning of the rack mounted needle blanks. The rack mounted needle blanks may be presented to various operations requiring organized presentation of needle blanks or needles. Such operations include grinding, channel forming, curving, needle blank cut-off, drilling, annealing, needle point forming, needle body forming, needle passivation, de-greasing, electro-polishing, washing, rinsing, drying, and coating with a lubricating substance, such as silicon mixture for example.
Annealing a portion of the needle blank prior to cut-off may be performed by introducing a flame and moving either the flame or the rack in a controlled fashion. One type of flame annealers, referred to as a channel flame annealer, uses a long fixed continuous burner tube. Fuel for the flame may be natural or bottled gas (propane or butane) mixed with air or compressed air. The burner tube may be a standard pipe or tubing with series or rows of drilled holes to provide flames. Wider holes provide a wider flame to anneal a wider portion of the needle blanks.
A timer controls duration of the flame, turning the flame on and off, or controls duration of the needles blanks in a continuous flame. This controls the heat transfer to the needle blank portion or anneal zone. A heat sink is used to confine the anneal zone. The duration of the flame may be controlled by electrically controlling a gas solenoid, which when opened, uses a pilot light or a piezo crystal to ignite the entire burner.
Alternatively, the duration of the needle blanks in the combustion zone may be controlled by moving the rack in and out of the combustion zone using conventional mechanical means. For example, a pivoting fixture receiving mechanism is used to lower the fixture mounted needle blanks into a position of engagement with the combustion zone. The duration of the engagement may be pre-set using a conventional timing mechanism. The timer setting may be realized by mechanical means, such as engagement of rotational means which applies a rotational action to the pivoting fixture receiving mechanism. The timer mechanism, through an air actuation means, could also allow the pivotal motion of the fixture receiving mechanism to be transferred through a cylinder means. This provides the proper duration of the needle blanks engagement with the combustion zone.
This channel flame annealer may be semi-automatic, where a micro-switch detects presence of the rack, actuates cylinders which clamp and locate the rack having needle blanks thereon, and automatically starts, times, and stops the anneal cycle.
In another flame annealer, referred to as a precision flame annealer, the burner itself is moved in and out of anneal zones of a batch or individual needle blanks mounted on the rack. The duration of annealing is controlled by a conventional timing mechanism. The burner may be a single small port burner or a multi-port long burner. The small burner may traverse the rack mounted needle blanks to heat one needle at a time in a very precise zone. Heat transfer is controlled by the speed of traverse. The burner may be moved by conventional means, such as a linear motor, a motor driven ball screw or timing screw, a motor with rack and pinon, or an air cylinder. This precision flame annealing may be semi-automatic, where loading the rack activates the anneal cycle.
In higher speed annealing, propane or similar fuels mixed with air may not burn hot enough. Instead, oxygen or other flammable gases, e.g., hydrogen, may be mixed with conventional fuels or methane, to achieve a more consistent and higher burning temperature. Despite increased speed, flame annealers are still slow and typically limited to 20 parts per minute.
Instead of mounting the needle blanks on a fixture or rack, the needle blanks may be mounted on the carrier strip 15 shown in FIG. 1. This increases annealing speed and needle production output. For a stationary flame, the strip mounted needle blanks may be continuously moved or indexed, pausing for a brief moment withing the flame. The limiting factor is again consistency and repeatability of the heat transfer. Using intense heat, anneal timing is critical for proper annealing. Due to the high intensity heat, even a slightly long anneal time leads to burning or melting the anneal zone portion of the needle blanks. Precise regulation of gas pressure or flame temperature is needed. This is difficult to achieve and still may not lead to a repeatable annealing. To improve annealing, a hydrogen gas generator is used and a small amount of alcohol is added as a flash suppressant.
The conventional flame annealers suffer from low speed. To increase annealing speed, laser flame annealers have been used. Laser high speed flame annealers provide a hot flame at a fairly precise anneal zone without the need for heat sinks. However, such annealers require a complex fuel supply and have some drift in the heat output. Other disadvantages include lack of feedback, presence of open flames, and difficulty in achieving repeatability.
Resistance annealers heat the anneal zone by passing a current therethrough. Current is passed through a pair of spaced electrodes clamped at two ends of the anneal zone. This current heats the wire potion 25 (FIG. 1) located between the clamped electrodes to the annealing temperature. Delivered heat to the anneal zone portion of the wire equals the product of time and the square of the current as shown in equation (1) : EQU H=I.sup.2 T (1)
where:
H is total heat; PA1 I is current; and PA1 T is time or duration. PA1 (a) setting a constant current amplitude and a current time; PA1 (b) providing a constant current for the set time to the needle through a switching device; and PA1 (c) varying a voltage drop across the switching device in response to a voltage drop change across the needle. PA1 connecting one terminal of the reference resistor to a power supply; PA1 generating a reference voltage using a reference voltage generator; and PA1 switching a switch to provide the reference voltage at another terminal of the reference resistor using an operational amplifier. Another step includes generating a current timing signal using a timing programmable logic controller. PA1 setting a current amplitude using a current amplitude setting device; PA1 setting the current time using a current duration setting device; PA1 clamping a pair of spaced apart electrodes on the wire or needle; PA1 generating an anneal signal from a system programmable logic controller indicating a completion of the clamping; PA1 measuring an amplitude of the constant current using a current detector; PA1 indicating a level of the detected current amplitude on a monitoring device; and PA1 stopping the annealing device when the indicated current level exceeds a predetermined level, or when a number of improper indications exceeds a predetermined number.
Conventional resistance annealers do not suffer from some of the flame annealer disadvantages. For example, soot or carbon build-up is eliminated. The annealer has dual clamping contacts which are cam operated. The anneal cycle is cam actuated. However, conventional resistance annealers operate at low speed, such as 10 parts per minute, and are limited to use with small wire sizes. This is because conventional resistance annealers have a maximum current of approximately 20 amps. In addition, no feedback is provided and proper annealing is manually checked, e.g., by viewing the color of the annealed wire. Color of the annealed portion ranges from gold, red, blue to silver. Determining the properness of annealing by viewing is imprecise and subjective.
In addition, current is monitored using an oscilloscope and adjusted manually with a potentiometer. The current duration is determined using an electro-mechanical timer having a resolution of 0.1 seconds. Such annealers are inaccurate, slow and require operator intervention. Conventional resistance annealers are complex, cannot operate at high speed, and suffer from inadequate precision and high failure rate of annealed wires or needles.
Other conventional resistance annealers measure voltage or temperature of the annealed wire to adjust the current therethrough. A resistance annealer which measures temperature is disclosed in U.S. Pat. No. 4,409,042 (Dornberger). Such annealers require complex reflectors and are not suitable for high speed annealing. Speed is limited due to the response time of temperature sensors. UK Patent Application GB 2 091 002 (Ash) discloses a wire annealer where wire resistance is measured as a representation of wire temperature. U.S. Pat. No. 3,746,582 (Gentry) also discloses wire annealing, wherein temperatures are controlled by varying the current through the spaced contacts for heating the wire portion located between the contacts. Other resistance annealers are disclosed in U.S. Pat. No. 3,842,239 (Ellinghausen), and U.S. Pat. No. 3,962,898 (Tillmann). These annealers are not suitable for high speed, high precision annealing.